Conventional conjugate gearing consists of rigid gears having unitary teeth rigidly formed thereon, which gears convey mechanical power by forces transmitted from the unitary teeth of one gear to the unitary teeth of a mating gear. As the gears rotate, the point of contact on any engaged tooth moves from tip to root or from root to tip of the tooth. This contact includes both a rolling component of motion and a sliding component of motion, the sliding component having a velocity which reverses direction as the point of contact crosses the meshing circle of the gear. (The meshing circle is determined by the center distance of the gears and their numbers of teeth, but does not depend on the geometry of the gears.) The sliding component produces sliding action between the teeth, resulting in friction, which in turn causes noise, wear, energy losses and thermal expansion of the gear teeth.
Due to the aforementioned thermal expansion, the tooth thickness of at least one member of a pair of mating gears must be formed slightly smaller than the space between the teeth of the opposed mating gear to accommodate said thermal expansion and prevent binding of the gear teeth. Because of this tolerance factor necessitated by thermal expansion, the gear teeth of a meshing pair engage on only one face of their tooth profiles. Also, in many instances, only one pair of teeth are engaged at any particular moment and both engaging teeth have concave profiles, thus generating highly concentrated contact (Hertzian) loading due to transmitted payload forces. Therefore, since only a fraction of the gear tooth faces are engaged and since intensive payload forces are acting on the tooth surfaces while the latter are engaged in the reciprocating sliding process, the load capacity of the gears is substantially limited. Enhancement of the load capacity is highly desirable in applications requiring transmission of high power at minimum transmission weights, such as for high performance vehicles and aerospace applications such as satellites, spacecraft and the like.
Typically, state-of-the-art toothed gears are machined from highly alloyed steel. The gears are then given special heat treatment to lend the teeth high core strength combined with great surface toughness and endurance. However, profile errors in the teeth caused by the machining process often lead to high intensity noise and increased friction between engaged faces of the mating teeth. Furthermore, lubrication is complicated by the in-cycle stoppage.
In many applications, the reduction of noise is crucial. For example, the most common means of detection of submarines is an underwater acoustic method in which sounds emitted by the various mechanical components of the vessel are detected. An important component of this emitted noise is produced by the gearing used in submarine machinery. By eliminating or greatly reducing the intensity of the sound, probability of detection by the acoustic method may be reduced.
Some attempts have been made in the prior art to reduce or minimize sliding friction between, and noise created by, the engaging faces of gear teeth. For example, U.S. Pat. No. 4,665,763 discloses a worm gear device in which the worm gear is configured as a wheel having a plurality of spheres captured in cages disposed thereabout. The worm is configured as a track which engages these spheres. In this manner, sliding contact between the worm gear and the worm is replaced by rolling contact, thus minimizing friction. However, such a worm gear system is difficult to fabricate and has not enjoyed widespread application. U.S. Pat. No. 3,448,631 discloses a gearing system in which sliding friction is eliminated by including a plurality of pressure pads on the gear teeth of the rack, each pad adapted to release a high pressure stream of hydraulic fluid to form a fluid bearing surface. Obviously, such a complicated lubrication system increases both the bulk and weight of the gear system and thus, negates the usefulness of the system in applications requiring minimum weight and bulk.
U.S. Pat. No. 4,184,380 issued to the inventor of the present invention, discloses a toothed gearing system in which a resilient coating is affixed to the engaging surfaces of the gear teeth, the coating having a thickness, shear resistance and coefficient of friction such that the sliding that would occur between uncoated teeth is taken up by shear deformation of the coating. However, pure shear deformation in the coating would materialize only in cases when curvature radii of the tooth surface are constant along the profile or segments thereof, such as is the case in conformal (Wildhaber-Novikov) or in clock gears. If the curvature radius of the profile is continuously changing, as in the most widely used involute gears, then a compression component is added to shear deformation in the coating. It increases resistance to deformation in the coating and reduces effectiveness of the method.
U.S. Pat. No. 4,543,841 discloses a gearing system in which a first gear wheel having a plurality of conventional, fixed teeth is driven by a second wheel having radially slidable teeth. In response to centrifugal force, the slidable teeth project from the periphery of the second gear wheel and engage and drive the first gear wheel. In this patent, each entire driving tooth is slidable but only in a radial direction. U.S. Pat. No. 4,373,925 discloses an elastic coupling comprised of two driving gears engaging a planet gear. This gearing system is adapted to maintain constant contact between gear teeth and, towards that end, includes radially slidable teeth. However, the teeth are not operative to eliminate sliding friction. U.S Pat. No. Re. 28,696 discloses a particular profile and shape of gear teeth designed to reduce sliding friction. This invention finds particular applicability to gears molded from dry bearing materials. It would be difficult to adapt to heavy duty gears which are cut rather than molded.
In short, none of the prior art patents discussed above serve to eliminate or minimize the sliding component of motion, as well as attendant friction and noise problems, in a completely satisfactory manner. The gearing systems disclosed in these references are either prohibitively expensive to produce, not suitable to heavy-duty applications, or not particularly effective in reducing sliding friction.
It will thus be appreciated that there remains an unfulfilled and long felt need for a tooth gear system of relatively simple design which may be employed to reduce or minimize sliding friction and attendant noise generated during the meshing engagement of the gears. The present invention addresses the inadequacies of the prior art by providing a conjugate gear with a three component tooth which acts to separate the sliding component of motion from the rolling component of motion, therefore reducing sliding friction, thermal expansion, and attendant noise levels. The herein disclosed conjugate gear also has the advantage of reducing material costs in that an interior or core portion of the gear can be fabricated of a strong, but possibly light, material, with surface portions thereof made of a material having high hardness and good wearing characteristics. These and other advantages of the present invention will be readily apparent from the drawings, discussion and claims which follow.